A polyimide powder for heat compression molding is produced by reacting an aromatic tetracarboxylic acid dianhydride with an aromatic diamino compound in an organic polar solvent to form a polyamide acid, which is then imidated to precipitate polyimide particles. The resulting polyimide powder may be used, for example, as a molding material or may be mixed with a filler to impart strength characteristics to molded products. In the latter case in which a polyimide powder and a filler are mixed to obtain a polyimide molding powder, a polyimide powder and a filler powder are dry blended in a mixer, etc., or each powder is separately dispersed in a dispersing medium and the dispersions are mixed, followed by removal of the dispersing medium (wet blending). According to these methods, the polyimide powder and filler can uniformly be mixed, but the resulting mixed powder is a mere mechanical mixture of the polyimide powder and the filler. However, since a polyimide powder itself is substantially infusible or insoluble, it does not sufficiently flow in the heat compression molding, particularly due to interference from the filler powder. As a result, molded products obtained from such a molding powder is extremely brittle. Moreover, they have a seriously poor appearance as not only lacking gloss but also having cracks or spots due to non-uniform distribution of the polyimide powder on their surface.
Japanese Patent Publication No. 22196/64 discloses a process for preparing a polyimide molding powder by suspending a fine metal powder or a metal oxide, etc. directly in a polyamide acid solution. However, this process produces a large-sized polyimide powder composed of a filler aggregate coated with a polyimide, from which only a very brittle molded product can be obtained. In other words, since the filler aggregate that is a mere assembly of particles is first subjected to destruction upon application of external force, the molded products are exceedingly brittle and thus have a small strength.
Under these circumstances, the conventional polyimide molding powders containing a filler can produce no more than brittle molded products having poor appearance with no gloss. Nevertheless, it is inconceivable to exclude the filler, taking into account the excellent characteristics imparted to polyimide molded products by the filler. It has been, therefore, keenly demanded to improve flowability of a polyimide molding powder containing a filler during the molding.